When the laser beam exits the resonator, its diameter is often too large and diffuse, and the beam itself may have inadequate power to be useful. Therefore, the laser beam is passed through a focusing lens to reduce its diameter, which increases its intensity and energy so that it is of more suitable size for manipulation and practicality. Its intensity, referred to as its power density (Pd) or irradiance (E) is defined as the energy delivered per unit area of incident tissue. It is measured in terms of wattage of laser per diameter of the beam. That is, Pd varies inversely with the square of the diameter of the laser beam, as follows:
Pd = (100 W)/d2, where W is the laser power in watts, and d is the diameter of the laser beam in centimeters (ie, 100W/cm2).
For a given wattage, a wide or unfocused beam has less penetration ability and is more useful for procedures such as skin resurfacing, vaporization of tissue, and coagulation of blood vessels. A focused beam penetrates to a greater depth and is more useful in procedures involving delicate cutting and volume ablation.